This invention relates generally to reducing emission of nitrogen oxides from combustion systems, such as boilers, furnaces and incinerators.
A group of air pollutants produced by combustion in boilers and furnaces include oxides of nitrogen, mainly NO and NO2. Nitrogen oxides (NOx) are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. Reduction of nitrogen oxides has been the focus of many technology development efforts.
In modern boilers and furnaces and other such combustion vessels, emissions of nitrogen oxides (NOx have been greatly reduced by the use of overfire air (“OFA”) technology. In this technology, most of the combustion air goes into the combustion chamber together with the fuel, but addition of a portion of the combustion air is delayed to yield oxygen lean conditions initially and then to facilitate combustion of CO and any residual fuel.
Selective Non-Catalytic Reduction (“SNCR”) technologies reduce NOx in combustion gas by injecting a nitrogenous reducing agent (“N-agent”), such as ammonia or urea, into the gas. The N-agent is injected at high temperature and under conditions such that a non-catalytic reaction selectively reduces NOx to molecular nitrogen. Reduction of NOx is selective because the molecular nitrogen in the combustion gas is not reduced, while the NOx is reduced by the N-agent.
The N-agent is typically released into flue gas that is within an optimum temperature range or window, such as between 1700 degrees to 2200 degrees Fahrenheit (930 to 1200 degree Celsius). The flue gas often has moderate to high carbon monoxide (CO) concentrations (0.2-1.0 percent). In some SNCR applications, the CO in flue gas chemically competes with the active species in the N-agent needed for NOx reduction. This competition reduces the effectiveness of the SNCR process and NOx reduction, and/or moves the optimum temperature window to lower temperatures.
Earlier SNCR techniques circumvented the CO problem by spraying large N-agent droplets into overfire air injected into the flue gas. As the OFA and flue gas steams mix, CO is oxidized and water in the droplets evaporates as the droplets are carried to cooler regions of the boiler. This process delays the release of the N-agent until the gas temperature has reached the optimal temperature window.
Large droplet N-agent systems have difficulties that can reduce their effectiveness such as: long droplet residence times in the flue gas, a tortuous flow path with obstructions for the droplets, and a narrow N-agent release temperature window. If the droplets are too small, they release the N-agent upstream of the optimal temperature window where the flue gas is still too hot and render the N-agent ineffective. Under these conditions, the N-agent can generate (rather than reduce) NOx. On the other hand, if the droplets are too large, a portion of the N-agent is released after the combustion gas has cooled below the optimal temperature window causing high ammonia concentrations (ammonia slip) in the flue gas outlet stream. Finally, there is a need for better SNCR techniques to address the problems raised by high CO concentrations in the flue gas near the droplet injection location.